Flexible Life History Traits: Shifts by Scarlet Gilia in Response to Pollinator Abundance.

Abstract

In response to reduced levels of pollinators and pollination, scarlet gilia, Ipomopsis aggregata, shift from their normal semelparous mode of reproduction to iteroparous reproduction. When pollinators were excluded, plants were 5.5 times as likely to produce an ancillary rosette as controls that received natural pollinator levels. Similarly, when flowers were removed to prevent fruit production, experimental plants were 7.8 times as likely to produce and ancillary rosette as unmanipulated controls exposed to natural levels of pollination. Although ancillary rosette production is correlated with both season and a decline in pollinators, when seasonality was experimentally eliminated as a variable, changes in pollinator abundance still resulted in a corresponding change in rosette formation. Results suggest that there is a threshold below which an individual can be expected to switch from semelparity to iteroparity; the threshold appears to be between 30 and 40% fruit set. Our experiments also demonstrate a trade-off between cloning and fruit production at the population level. Herbivory had no effect on these life history traits. Plants clipped to simulate natural herbivory by deer and elk showed no change in rosette formation relative to undamaged controls. These and other data demonstrate that pollinators are limiting and suggest that pollination is the primary factor involved in these life history trait shifts by scarlet gilia. Midseason shifts in life history traits permit plants to adjust to current lows in pollinator services by producing an ancillary rosette that can then flower in a subsequent year when pollinators may be less limiting.